Binary material field emitter structure

ABSTRACT

A field emitter structure comprises a body of a binary eutectic alloy wherein thin filaments of the minor component of the alloy are embedded in, and a plurality of the thin filaments project above, a surface of a matrix enriched by the major component of the alloy thereby providing a highly effective and inexpensive non-thermionic source of electrons for a variety of vacuum and other applications.

United States Patent n 1 Brody 1March 13, 1973 BINARY MATERIAL FIELDEMITTER STRUCTURE Thomas P. Brody, Pittsburgh, Pa.

Westinghouse Electric Corporation, Pittsburgh, Pa.

July 29, 1970 Inventor:

Assignee:

Filed:

Appl. N0.:

US. Cl. ..313/309, 313/336, 313/351 Int. Cl. ..II0lj l/02 Field ofSearch ..313/309, 336, 351

References Cited UNITED STATES PATENTS 7/1966 Shroff 7/1969 Shoulders etal. ..3l3/35l X 9/1969 Arthur et a1 ..313/309 X 9/1970 Frankland..313/351 X OTHER PUBLICATIONS Dranova et a1., High-current PulsedField-Emission Cathode, Chem. Abstracts, Vol. 70, June 30, 1969 No.ll9308s.

Dudley et al., Rare Earth Oxide Cermet Cathodes,"

Chem. Abstracts, Vol.58, 1963, No. 6295g.

Cline, Multineedle Field Emission from the Ni-W Eutectic, Journal ofApplied Physics, Vol. 41, No. 1, Jan. 1970, pp. 76-81. Gifford et al.,Thermionic Emitters Consisting of BaQ-UO Dispersed in a Tungsten Matrix,Journal of Appl. Physics, Vol. 38, No. 5, April 1967, pp. 2261-2268.

Garber, R. 1.; High Current Field-Emission Cathode, Translation fromPriboryi Tekhnika Eksperimenta, No. 1, pp. 196-198, February 1969.

Primary ExaminerDavid Schonberg Assistant Examiner-Paul R. MillerAttorney-Fr Shapoe and C. L. Menzemer [57] ABSTRACT A field emitterstructure comprises a body of a binary eutectic alloy wherein thinfilaments of the minor component of the alloy are embedded in, and aplurality of the thin filaments project above, a surface of a matrixenriched by the major component of the alloy thereby providing a highlyeffective and inexpensive non-thermionic source of electrons for avariety of vacuum and other applications.

5 Claims, 3 Drawing Figures PATENTEDMARIB 1975 3,720,856

I4 22 li ll |H|H i '6 5/ l2 s WITNESSES: INVENTOR OSWMRQ'QCh' I ThbmosP. Brody gwaihwfii BY WWW ATTORNEY BINARY MATERIAL FIELD EMITTERSTRUCTURE BACKGROUND OF THE INVENTION 1. Field of the Invention Thisinvention relates to non-thermionic electron sources, and in particular,to a field emitter structure suitable for use as a non-thermionic sourceof electrons for a variety of vacuum and non-vacuum applications.

2. Description of the Prior Art The provision of non-thermionic (cold)electron sources for a variety of scientific and commercial applicationsis highly desirable. Heretofore one field emission cathode was producedby spot welding 40 tungsten wires to form a comb structure. Later,multiple-needle field emission cathodes were produced by growingmolybdenum whiskers on a substrate. Other prior art endeavors have beencentered on thin film sandwich techniques and related field emitterstructures which have been extensively investigated. However, suchstructures are difficult to fabricate, sensitive during operation andrelatively fragile. In particular, the sensitivity of such structuresallows for easy destruction by localized hot spots during normaloperation. Additionally, the sandwich type structure of a cathode tendsto show a progressive deterioration in performance as a result ofnon-reversible changes in the exit metal layer. Field emitter structuresconsisting of a single emitter point or a small number of emitter pointshave a very limited emission capability. Consequently, a demand for aninexpensive, extended area non-thermionic source of electrons exists.

More recently, H. E. Cline in his paper Multineedle Field Emission fromthe Ni-W Eutectic," Journal of Applied Physics, volume 41, No. 1,January 1970, described a method of making a multineedle field emitterstructure from an alloy of nickel and tungsten of essentially eutecticcomposition. H. E. Cline does not teach any specific geometricalstructural orientation of the multineedle field emitter and isrestricted to the nickel-tungsten binary eutectic alloys.

Field emitter structures consisting of a single or a small number ofeutectic points have a limited emission capability. A demand for aninexpensive, extended area non-thermionic source of electrons exists.

SUMMARY OF THE INVENTION In accordance with the teachings of thisinvention, there is provided a field emitter structure comprising a bodyhaving a surface and comprising a material consisting essentially of alamellar microstructure of an ordered structure of thin filaments of theminor component of said lamellar microstructure being substantiallyperpendicular to the surface and embedded in a matrix of the majorcomponent of the material. The matrix comprises a cermet or a binaryeutectic alloy of chromium copper, or alloys of tungsten, molybdenum ortantalum with silicon. A plurality of the thin filaments project out ofthe body above the surface to a predetermined height and are orientedwithin i20 of the vertical axis of the body.

DRAWINGS FIG. 1 is a greatly enlarged top plan view of a field emitterstructure made in accordance with the teachings of this invention;

FIG. 2 is a greatly enlarged elevation, partly in crosssection, of thefield emitter structure of FIG. 1 taking on the cutting plane Il-II; and

FIG. 3 is a fragmentary magnified view of a single etched filament.

DESCRIPTION OF THE INVENTION With reference to FIGS. 1 and 2, there isshown a field emitter structure, or cathode, 10 suitable for use as anon-thermionic source of electrons. The structure 10 comprises a body 12comprising a binary eutectic alloy or a cermet having a fibrous, orlamellar, microstructure wherein a dense ordered structure of thinfilaments 14 comprising the minor component of the alloy or the cermetis embedded in and projects above the surface 18 of a matrix 16 enrichedby the major component of the alloy or the major component of the cermetrespectively. Examples of suitable binary eutectic alloys for comprisingthe body 12 are copper-chromium, tungsten-silicon, molybdenum-silicon,and tantalum-silicon. An example of a suitable cermet is uraniumdioxide-tungsten. The alloy composition by weight percent may vary about1 percent from the eutectic composition but a preferred range is :54;weight percent from the eutectic composition. The tungsten may comprisefrom 5 to 15 weight percent of the uranium dioxide-tungsten cermet. Inthe cermets the thin fila ments 14 extend from the face surface 18 tothe rear surface 20.

The body 12 is made by cooling a molten mass of binary eutectic alloy ora cermet material at a predetermined rate from one end to the other tocause progressive solidification in order to produce the fibrous, orlamellar, microstructure wherein the thin filaments 14 are substantiallyparallel to the vertical axis of the resulting ingot, for example,within i20 of the vertical axis. By controlling the composition of thealloy or cermet, as well as the cooling rate and impurity content, oneis able to control the number, distribution and diameter of the thinfilaments 14. Control of the composition and the cooling rate to do thisis within the competence of one skilled in the art. After the alloy orcermet has been preferentially solidified by slow cooling to form aningot of rod-like shape, a transverse section is removed from the ingotand a first major top surface 18 of any desired shape is prepared bypolishing. The top surface 18, if substantially flat, is within i20 ofbeing perpendicular to the filaments l4 and preferably is substantiallyperpendicular to the ordered orientation of filaments 14. A second majorrear surface 20 is also prepared by a polishing technique and may beflat and substantially parallel to the surface 18 or it may be preparedto a predetermined curvature.

A plurality of the thin filaments 14 must extend entirely through thebody 12 when the material is a cer-' met. When the body 12 comprises abinary eutectic alloy, the thin filament 14 need not extend entirelythrough the body 12 since the matrix 16 will be electrically conductive.

After initial preparation of the body 12, selective etching of the topsurface 18 removes substantially only the matrix 16 from about thefilaments 14, leaving the filaments projecting out of the surface 18 asshown in FIG. 2. As prepared, many of the etched filaments have bluntends although others appear to taper to a point with about one-tenth thediameter of the average filament. The prepared body 12 at this point issuitable for use as a highly effective field emission structure 10.However, the efficiency of the structure may be further increased byselectively etching the tips of the exposed filaments 14 to producefilaments 22 having a tip radius R as shown in FIG. 3. The fieldemission of the tips of the filaments 22 increases approximatelyinversely with the tip radius, while the emission goes up exponentiallywith the field. Filaments 22 having a tip radius R of from 3000A to4000A are suitable for use in partial vacuum of the order of l to 50 cm.of Hg, while those filaments 22 having a tip radius R of from 100A to200A are suitable for use in air or gas at atmospheric pressure. If thematerial comprising the body 12 is prepared properly extremely smalldiameter filaments result so that little or no selective etching isrequired for shaping the tips of such very thin filaments. Even in thisinstance, however, the structure 10 is not as an efficient emitter asthat prepared by selectively etching all the filaments. In any event,the structure 10 does provide an economical and effective nonthermionicsource of electrons.

The exposed height, h, of the filaments 10 should be a minimum of atleast 10 to 15 microns in order to assure a good source of electronemission. If the filaments 14 are spaced closer together they mutuallyshield each other thereby decreasing the efficiency of the field emitterstructure 10. Therefore, it has been determined that the distance, d, inmicrons between any two adjacent filaments 14 should be at least of theorder of 4 VH2 where h is in microns and R is the top radius inangstroms. A close to optimum structure 10 has been determined as beingone where the filaments 14 extend approximately 100 microns in heightabove the top surface 18 and are spaced apart from each other a distancegiven by the above expression, namely 40m In order to more fullydescribe this invention, particular reference will be made to astructure 10 wherein the body 12 comprises a chromium copper alloywherein chromium is from 9% percent to 2 percent by weight of the alloy,which upon melting and controlled progressive longitudinalsolidification forms five filaments of chromium. A suitable etchant forselectively etching the copper matrix 16 from about the chromiumfilaments 14 is nitric acid.

More particularly, an ingot of a copper-chromium eutectic alloycontaining 1.5 weight percent chromium was cast, rolled, and swaged intoa 0.2 inch diameter rod. The alloy contained approximately 200 parts permillion of impurities. The swaged rod was encapsulated in a high purity,99.9 percent, alumina tube and regrown in a vertical Bridgman furnace ata rate of h inch per hour. The molten metal in the furnace was at 1200"C and there was a temperature gradient of the order of 150 C per inch atthe interface of the newly regrown rod and the initial swaged rod. Thevertical regrowth of the rod caused a lamellar structure characterizedby a fine, uniform distribution of chromium whiskers, or filaments 14,axially oriented in substantially the direction of the rod axis.

The grown rod was cut perpendicular to its axis to produce a substratewafer about l/l6 inch in thickness. The opposed major surfaces of thesubstrate wafer were polished and one surface was exposed to a 50percent solution of nitric acid for 20 seconds to selectively etch thecopper rich matrix 16 away from the chromium filaments 14. The result ofthis selective etching was to produce filaments l4 protruding about 0.1millimeter from a surface 18. The filaments 14 were not further etched.The structure 10 was mounted in a holder comprising an electricallyinsulating material, polytetrafluoroethylene with an electrical contactwas affixed to the rear surface 20, and the assembled components placedin a high vacuum system for emission studies. The separation between theemitting surface, that is, the plane of the tips of the filaments 14,and a plain stainless steel anode was arranged so that it could bevaried and could be measured to :1 mil.

Prior to testing the structure 10, a polished flat stainless steel waferwas mounted in the test fixture to act as a cathode with a space of 10mils. between the anode and cathode, and 10 KV was applied to the anode.No observable emission was noted under a high vacuum. It was determinedthat leakage currents, if they existed, were less than 1 X 10' amps andtherefore were neglected. The emission currents were measured with anelectrometer.

In the high vacuum system, the structure 10 as processed, waselectrically connected to a linear motion feed-through of a UHV system,the stainless steel anode being disposed near and parallel to thestructure 10, and the system evacuated to about 10 Torr and voltageapplied between the emitter structure 10 and the anode. The resultingemission current was measured with the electrometer as a function ofaccelerating voltage and plate separation. Test results obtained were asfollows:

TABLE I CONSTANT VOLTAGE OF 500 VOLTS APPLIED Separation between anodeand cathode mils) Emission Current (p. A)

The results as shown in Table 1 indicate that emission currents are notgreatly affected by the separation distance between cathode and anode.

TABLE II CONSTANT SEPARATION BETWEEN CATHOD E AND ANODE 10 MILS Voltage(volts) Emission Current (p. A)

the cathode, or field emitter structure, and the anode of up to andincluding 56 of an inch.

As a control, a polished stainless steel plug of the same geometry asthe field emitter structure was inserted in the test apparatus in placeof the field emitter structure. No emission was detected at any settingpreviously used, and no leakage currents were observable at an ammetersetting of ampere, which was full scale deflection. Therefore, anyemission current, if there be any at all, was necessarily below 10-"ampere.

Emission currents as high as 250 microamperes are obtainable with thefield emitter structure described heretofore. These currents have beenmaintained for days without degradation.

Field emitter structures embodying the chromiumcopper binary eutecticalloy compositions have been found to resist deterioration afterexposure to air and yielded the same emission currents upon retesting inthe high vacuum system as were obtained during previous testing in thesame high-vacuum system.

When a cermet such, for example, as uranium dioxide-tungsten comprisesthe body 12, electrical contact is made to the bottom ends of thefilaments 14 by plating the bottom surface 20 with a layer 24 of anelectrically conductive metal such as copper. The layer 24 provides ameans of applying an electrical potential to the filaments 14 whichextend through the entire body 12. Since the ordered structure offilament growth provides substantially all filaments grown the completelength of the ingot, very few, if any, of the filaments 14 will not beconnected electrically by the layer 24. The layer 24 is not needed forthe binary eutectic alloy materials since the matrix 16 of such alloyscomprises an electrically conductive material.

I claim as my invention:

1. A field emitter structure comprising a body having a surface andcomprising a material consisting essentially of a lamellarmicrostructure of an ordered structure of thin filaments of the minorcomponent of said lamellar microstructure substantially perpendicular tothe surface and embedded in a matrix of the major component of saidmaterial;

a plurality of the thin filaments projecting out of said body matrix apredetermined height above said surface, the thin filaments being withini20 of the vertical axis of said body, said material comprising saidbody being a binary eutectic alloy of copperchromium wherein the majorcomponent chromium varies from the eutectic alloy composition by up to:1 weight percent.

2. The field emitter structure of claim 1 wherein said filaments arespaced apart from each other at least of the order of 4 mmicrons where his the height in microns that the filament projects above the surface ofthe matrix, and R is the radius in angstroms of the tip of the filament.

3. The field emitter structure of claim 2 wherein said filaments projectabove said one of the two opposed surfaces at least 10 microns.

4. The field emitter structure of claim 3 wherein R is at least about A.

5. The field emitter structure of claim 2 wherein said filaments arespaced 100 microns apart from each other and each of the filamentsprojects 100 microns above said surface.

1. A field emitter structure comprising a body having a surface andcomprising a material consisting essentially of a lamellarmicrostructure of an ordered structure of thin filaments of the minorcomponent of said lamellar microstructure substantially perpendicular tothe surface and embedded in a matrix of the major component of saidmaterial; a plurality of the thin filaments projecting out of said bodymatrix a predetermined height above said surface, the thin filamentsbeing within + or - 20* of the vertical axis of said body, said materialcomprising said body being a binary eutectic alloy of copper-chromiumwherein the major component chromium varies from the eutectic alloycomposition by up to + or - 1 weight percent.
 2. The field emitterstructure of claim 1 wherein said filaments are spaced apart from eachother at least of the order of 4 Square Root hR microns where h is theheight in microns that the filament projects above the surface of thematrix, and R is the radius in angstroms of the tip of the filament. 3.The field emitter structure of claim 2 wherein said filaments projectabove said one of the two opposed surfaces at least 10 microns.
 4. Thefield emitter structure of claim 3 wherein R is at least about 100A.